For this year’s lists, the EWG reported that 65 percent of the thousands of non-organic produce samples analyzed by the Agriculture Department tested positive for pesticide residue. “The USDA washes and peels the produce items that it tests and they still find pesticide residues on 65 percent of the samples,” Alex Formuzis, a vice president of EWG, told CBS News.

For example, every sample of imported nectarines and 99 percent of apple samples tested positive for at least one pesticide residue. A single grape sample contained 15 pesticides. In contrast, only 1 percent of avocado samples showed any detectable pesticides and none of the fruits and vegetables on the Clean Fifteen list tested positive for more than four pesticides.

Pesticides have been linked to developmental problems in children and can affect the nervous system as well as disrupt hormones in adults, according to the U.S. Environmental Protection Agency. Some studies have linked pesticide exposure to Alzheimer’s and Parkinson’s disease.

Although organic produce isn’t sprayed with pesticides, it’s typically costlier than conventional fruits and vegetables. For those on a budget, go for organic versions of the top offenders on the Dirty Dozen list and use the Clean Fifteen list to choose conventional produce with the lowest chemical levels.

Gary Adamkiewicz, senior research scientist in environmental health at the Harvard School of Public Heath, suggests the following guidelines:

Contamination can be skin deep: Since some residues are on the surface only, go organic when you will be consuming the entire fruit, skin and all (strawberries and apples). If the fruit has a thick skin or peel that is discarded (bananas and pineapples), go conventional.